In an electric system that supplies commercial electric power to consumers, a tight balance between electric power supply and demand in which the requirements for commercial power come close to exceeding supply generally occurs several times a year. As a result, power companies have taken countermeasures such as providing reserve power plants to relieve the tight balance between electric power supply and demand. However, this type of countermeasure entails the problem of the causes an increasing the cost of operating the electric power system resulting from the high cost of plant maintenance simply in order to relieve the tight balance between electric power supply and demand that occurs only several times a year.
In this regard, a technology known as “demand response” has been receiving attention due to its ability to relieve the tight balance between electric power supply and demand at a low cost. Demand response is a technology for avoiding a tight balance between electric power supply and demand at a low cost by communicating to consumers, in time periods in which the occurrence of tight balance between the electric power supply and demand can be anticipated, a demand response signal such as an electricity price signal indicating the imposition of peak time period charges (Critical Peak Pricing: CPP) in which electricity charges are higher than normal or a power-saving command indicating a demand to conserve power to cause consumers to change the temperature settings of air conditioners or the brightness of lighting apparatuses and thus lower the power consumption of electrical apparatuses. With demand response, a situation in which the tight balance between the supply of electric power and demand can be relieved without using reserve power plants, and thus the predicament of a tight balance between the electric power supply and demand can be relieved at low cost, and the operating costs of the electric power system can consequently be reduced.
In recent years low-cost high-capacity storage batteries have been introduced, as exemplified by lithium-ion storage batteries, and with this development, high-capacity storage batteries are beginning to be used in residences. Because lithium-ion batteries are being applied to electric vehicles in particular, even lower costs due to mass production can be expected with the popularization of electric vehicles, with the result that the popularization of lithium batteries in households is expected to advance.
A high-capacity storage battery in a residence is typically: used for storing electric power generated by a power plant such as a photovoltaic power generator, used as an auxiliary power supply during a commercial power outage, or used for economizing electricity charges. In such cases, the storage battery is controlled so as to undergo charging by commercial electric power or by electric power that is generated by power generation equipment such as a photovoltaic power generator and at other times is controlled to discharge electricity. For example, the storage battery is controlled so as to be charged during the transmission of commercial power and to be discharged during power outages, to be charged during periods in which electricity charges are low in a time-of-use rate system, and to be discharged during periods in which electricity charges are high (refer to Patent Document 1).
The reverse power flow of electric power that is discharged from a storage battery to the electric power system is currently not allowed, but in the future when this reverse power flow is permitted and the reverse-flow electric power is purchased by an electric power company, it is expected that the spread of high-capacity storage batteries to residences will increase.